Study On Soil Factors Affecting Permeability Of High Clay Dredger Fill Under Step Vacuum Preloading

Dredger fill is a kind of filling soil formed by hydraulic flushing and filling mud sand.It is one of the common artificial filled soils along the coast of our country.The main purpose of dredger fill is to fill some areas near sea or lake with mud by using high-pressure mud pump,so that it can be used as engineering foundation.However,the drainage consolidation of dredger fill is a great challenge due to the high content of organic matter,high water content,high soluble salt content and high compressibility of dredger fill soil.At present,the commonly used reinforcement technology is vacuum preloading.Although vacuum preloading can shorten the initial consolidation period,under the action of strong and continuous vacuum suction,fine particles in soil migrate to the vicinity of drainage plate under the action of vacuum suction and moisture in pore,blocking the orifice of drainage plate or accumulating near drainage plate,which makes the permeability of drainage plate worse or even lost.Especially when the clay content in soil is high,the siltation effect near the drainage plate is more obvious.On the basis of previous studies,it is known that the step vacuum preloading method can greatly reduce the migration of fine particles and effectively ensure the drainage consolidation of soil.In the process of consolidation by step vacuum preloading,the soil factors have been changing dynamically with time and space.The change of soil factors is the fundamental reason for the change of seepage properties of soil,and further affects the consolidation effect.Therefore,it is necessary to study the soil factors related to permeability of high clay dredger fill under step vacuum preloading,and to analyze the change law of permeability of soil during consolidation process,so as to clarify the consolidation mechanism,which could provide reference and basis for subsequent optimization of consolidation scheme.Based on the project of the National Natural Science Foundation of China "Study on time effect of microstructure and consolidation model for high clay dredger fill consolidated by step vacuum preloading ",a laboratory model test was carried out to reinforce the high clay Drainage Fill by step vacuum preloading.Through data monitoring,physicochemical experiment,permeability experiment and micro-structure parameter data,the spatial and temporal regularity of soil properties in different parts of the step vacuum preloading process is analyzed,and the correlation between porosity,pore solution concentration,clay content and soil permeability coefficient are discussed,so as to further analyze the change of soil permeability in the step vacuum preloading process.It mainly includes the following research contents:The granulometric composition of Tianjin dredger fill is mainly clay and silt,in which clay is dominant and belongs to high-clay dredger fill;quartz content is the highest in mineral composition,the second is illite mixed layerand content,the soil is hydrophilic;the soluble salt content of soil sample is higher,and the soil belongs to low liquid-limit clay.The monitoring data of pore water pressure and settlement during the consolidation process of step vacuum preloading show that with the increase of vacuum suction,the cumulative settlement at the center and side walls of soil has been increasing,and the total settlement at the center is less than that at the side walls.The pore water pressure decreases gradually,and the pore water pressure near the drainage plate is always lower than that at the side wall.The drainage speed changes in stages.At the beginning of gravity sedimentation and siltation,and at the beginning of each stage of vacuum pressure,the drainage speed is faster,and then slows down.The material composition and physicochemical properties of the soil at different locations in the test barrel during the consolidation process of step vacuum preloading indicate that the water content decreases gradually as the cumulative time of vacuum preloading increases,and decreases to below the liquid limit;the water content of the upper surface of the center is the lowest,the water content of the lower surface of the side wall is the highest,and tends to be the same at the end of 80 kPa vacuum preloading.At the end of the experiment,the clay content and pore solution concentration near the drainage plate were higher.The permeability test data of the soil during the consolidation process of step vacuum preloading show that the permeability coefficient of the soil decreases gradually with the increase of the accumulated time of vacuum preloading.The change of pore water type,particle migration and pore solution concentration will affect the change of percolation characteristics of pore water in the dredger fill.Qualitative and quantitative analysis of soil microstructures in the process of consolidation by step vacuum preloading shows that under the action of vacuum pressure,the way of particle accumulation changes to a more stable structure,the average pore size decreases with the increase of vacuum preloading cumulative time,and the fractal dimension of pore morphology decreases with the increase of time.The change of soil microstructure affects the change of permeability.The time grey correlation theory is used to analyze the correlation between permeability coefficient and seven factors,including porosity,macropore,medium pore,small pore,micropore,pore fractal dimension,clay content and pore solution concentration.The results show that the greatest correlation with permeability coefficient is the content of macropore,followed by clay content.